The Role of Ambient Turbulence in Plasma Instabilities Driven by Thermal Ions in the Solar Wind

Plasma instabilities driven by distribution functions of thermal ions are responsible for a number of physical processes in the solar wind. The description of the instabilities is usually based on the assumption of a uniform and stationary background. This assumption can be readily violated by the ambient turbulence which coexists with the instabilities. The turbulence is omnipresent as it transports the energy of very large-scales fluctuations generated by the Sun to much smaller kinetic scales. We investigate the effect of the ambient turbulence on the mirror and proton-cyclotron instabilities in a proton-alpha particle plasma. We perform three-dimensional hybrid simulations with particle-in-cell ions and a quasi-neutralizing electron fluid. The instabilities are driven by the protons with temperature perpendicular to the mean magnetic field larger than the parallel temperature. We compare the properties of the instabilities to the case of a uniform and stationary background and the same average temperature anisotropy and plasma beta. We find that the turbulence results in a modification of the proton distribution function, which contributes to the properties of the instability. Thereby, the turbulence starts to affect the instability before the unstable waves are excited. The initial growth rates of the mirror mode are close but the saturation level is significantly reduced when the turbulence is present. The saturation level of the proton-cyclotron mode is not affected by the turbulence as strongly.